cd: re-enable release signing

Description of issue or feature request:
https://github.com/theupdateframework/python-tuf/pull/1946 adds a CD workflow to release build artifacts on PyPI and GH upon successful completion of the CI workflow for a pushed release tag.

The PR also removes instructions from RELEASE.md to gpg sign release artifacts and add them to the GitHub release assets as part of the previously manual release process. However, the installation docs still mention release signatures.

Current behavior:
No instructions / release process integration to sign release artifacts

Expected behavior:
Add instructions to sign release artifacts and integrate with release process

Ideas:

• quick-fix 1: sign in GitHub CD action
• quick-fix 2: sign locally and upload signatures to release assets manually (bonus: integrate signing/uploading with verify_release script)
• long-term fix: e.g. in-toto (https://github.com/theupdateframework/python-tuf/issues/529)

Beginne mit dem durch PR 1946 eingeführten CD-Workflow und den Release-Anweisungen in RELEASE.md; vergleiche sie mit dem Abschnitt verify-release-signatures der Installationsdokumentation. Prüfe das verify_release-Skript als möglichen Integrationspunkt und ermittle anschließend, welcher Signaturansatz genehmigt ist. Als erledigt gilt die Aufgabe, wenn die Release-Artefakte Signaturanweisungen enthalten und die Signierung in den Release-Prozess integriert ist.